High strength Al-Zn-Mg alloy for making shaped castings including vehicle wheels and structural components

ABSTRACT

There is claimed an alloy composition for castings, especially vehicle wheels. This cast composition is chromium-free, zirconium-free and consists essentially of: about 4-7 wt. % zinc, more preferably, about 4-5 wt. % zinc; about 1.2-1.8 wt. % magnesium; at least one of about 0.3-0.6 wt. % manganese and 0.1-0.25 wt. % copper; up to about 0.25 wt. % iron; and up to about 0.25 wt. % silicon, the balance aluminum and incidental impurities. Products made from this alloy exhibit fatigue properties equal to or greater than those for a 6061-T6 forging, typically about 850,000 cycles at about 12.5 ksi; and about 330,000 cycles at about 20 ksi. They also exhibit a typical T6 tensile yield strength of about 50 ksi or higher.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation in part of U.S. application Ser. No. 09/366,039, filed on Aug. 2, 1999, which is a continuation in part of U.S. application Ser. No. 09/211,558, filed on Dec. 15, 1998 (now abandoned), the latter application having claimed the benefit of U.S. Provisional Application Ser. No. 60/069,932, filed on Dec. 17, 1997, all disclosures of which are fully incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] This invention provides an improved aluminum casting alloy that possesses strength properties comparable to wrought 6061-T6 (Aluminum Association designation) alloy products, yet with good fatigue life strength, corrosion and stress corrosion resistance. On a preferred basis, this alloy should also have reasonable castability and be able to exhibit a bright finish after polishing, something which cast alloys like A356 cannot provide. It is also preferred that castings made from this alloy composition be suitable for further plating with chrome and/or nickel, or capable of being subsequently coated with polymer layers for cosmetic and/or corrosion resistance purposes.

[0003] Numerous aluminum-zinc alloys have been patented over the years. These include: U.S. Pat. Nos. 2,146,330, 2,993,783, 3,306,717, 3,322,533, 3,388,987, 3,876,474, 4,636,357, 5,334,266 and 5,453,244 and British Patent No. 627,968. There are also high levels of zinc in the aluminum alloys of U.S. Pat. Nos. 2,146,331, 2,261,210, 2,290,017, 3,343,948, 4,711,762 and 4,866,479.

SUMMARY OF THE INVENTION

[0004] The above objectives of this invention are achieved by a chromium-free, zirconium-free, cast alloy composition which contains: about 4.0-7.0 wt. % zinc, more preferably about 4.0-5.0 wt. % zinc; about 1.2-1.8 wt % magnesium; at least one of about 0.3-0.6 wt. % manganese and/or about 0.1-0.25 wt. % copper; average impurity level concentrations of silicon and iron, less than about 0.25 wt. % each and more preferably less than about 0.15 wt. % Fe and Si, but no purposeful additions of either element thereto. The balance of said alloy is aluminum and incidental impurities. In other words, this new generation casting alloy composition contains no purposeful additions of such elements as chromium, zirconium, nickel, lithium and vanadium, thereby distinguishing it from other known casting alloys. Particularly, it has been observed that when small amounts of such elements as Cr and/or Zr were added to the aforementioned composition, no positive property effects were observed. With the foregoing composition, however, castings made from this alloy are able to achieve a typical T6 yield strength of about 50 ksi or higher.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0005] All component percentages herein are by weight percent unless otherwise indicated. As used herein, the term “substantially free” means that no purposeful additions of that alloying element were made, but that due to the presence of impurities and/or leaching from prolonged contact with manufacturing equipment, trace quantities of such elements may, nevertheless, find their way into the final alloy product so cast according to the present invention.

[0006] The term “minimum” strength (or other measurable property/characteristic) refers to that level which can be guaranteed such that about 99% of the product is expected to conform with 95% confidence at that level using standard statistical methods. “Typical” strengths tend to run a little higher than the minimum guaranteed levels associated with plant production but they are a worthwhile approximation when insufficient commercial values exist with which to perform a standard statistical calculation. Typical strengths also serve to illustrate an invention's relative improvement in strength when comparing same to established strength values of the prior art. This invention, for example, is able to achieve via its cast alloy composition typical T6-aged, tensile yield strengths of about 50 ksi or higher, often greater than about 53 ksi.

[0007] When referring to any numerical range of values herein, such ranges are understood to include each and every number and/or fraction between the stated range minimum and maximum. A range of about 4.0-7.0 wt. % zinc, for example, would expressly include all intermediate values of about 4.1, 4.2 and 4.3%, all the way up to and including 6.95, 6.97 and 6.99% Zn. The same applies to each other numerical property and/or elemental range set forth herein.

[0008] The alloy composition of this invention can be used in sand or permanent molds, or subject to other comparable casting processes. Parts made from this alloy can be solution heat treated, such as for 4-16 hours at one or more temperatures between about 450-590° C., then water quenched before being preaged at room temperature for at least 24 hours, followed by artificial aging for 1 to 24 hours at one or more temperatures between 120-190° C., in a single or multi-step process. One representative casting made from the foregoing alloy, with proper processing control, exhibited an ultimate tensile strength (UTS) in the range of 58-65 ksi, a typical tensile yield strength (YS) in the range of 53-60 ksi, and an elongation value of about 13-16%. These alloy products are also able to survive a 7 day, boiling salt water test at 18 ksi. Fatigue tests conducted on cast samples from this alloy composition showed 850,000 cycles at 12.5 ksi, equivalent to 6061-T6 forgings, and 330,000 cycles at 20 ksi, the latter being three times greater than the average observed for 6061-T6 forgings. With those properties, this casting alloy composition is suitable for manufacturing into vehicular structural components and wheels, especially cast wheels for mid-sized trucks. Representative examples of the structural components cast with the invention alloy include, but are not necessarily limited to, vehicular control arms, steering knuckles and differential carriers.

[0009] Distinct differences often exist between cast and wrought aluminum alloy products. Registrations for the respective compositions are separately addressed by the Aluminum Association with cast alloys being designated in three digits as compared to the four digit designations for wrought alloy products. Not all wrought alloy compositions have a cast equivalent either. Sometimes subtle differences enable one alloy product to be made by only one of these principal methods and not the other. Further processing, SHT, quenching, thermo-mechanical aging, etc., does not always translate well between product forms. For example, in U.S. Pat. No. 3,790,219, it was observed that cast Al alloy wheels may not withstand the stresses of their wrought “equivalents”, wheels which have been shaped by pressing or spinning from rolled sheet, or forged from a cast billet.

[0010] Having described the presently preferred embodiments, it is to be understood that this invention may be otherwise embodied in the scope of the claims appended hereto. 

What is claimed is:
 1. A shaped casting made from a chromium-free, zirconium-free, aluminum alloy exhibiting improved fatigue performance and a typical T6 yield strength of about 50 ksi or higher, said casting consisting essentially of: about 4-7 wt. % zinc, about 1.2-1.8 wt. % magnesium, at least one of about 0.3-0.6 wt. % manganese and 0.1-0.25 wt. % copper, up to about 0.25 wt. % iron, and up to about 0.25 wt. % silicon, the balance aluminum and incidental impurities.
 2. The shaped casting of claim 1 wherein said alloy contains about 4-5 wt. % zinc.
 3. The shaped casting of claim 1 wherein said alloy contains less than about 0.15 wt. % silicon and less than about 0.15 wt. % iron.
 4. The shaped casting of claim 1 wherein said alloy has fatigue properties equivalent to or greater than: about 850,000 cycles at about 12.5 ksi; and about 330,000 cycles at about 20 ksi.
 5. A shaped casting aluminum alloy which has fatigue properties equivalent to or greater than about 850,000 cycles at about 12.5 ksi; and about 330,000 cycles at about 20 ksi and a typical T6 yield strength of about 50 ksi or higher, said casting being chromium-free, zirconium-free and consisting essentially of: about 4-7 wt. % zinc, about 1.2-1.8 wt. % magnesium, at least one of about 0.3-0.6 wt. % manganese and 0.1-0.25 wt. % copper, up to about 0.25 wt. % iron, and up to about 0.25 wt. % silicon, the balance aluminum and incidental impurities.
 6. The alloy of claim 5 which contains about 4-5 wt. % zinc.
 7. The alloy of claim 5 which contains less than about 0.15 wt. % silicon and less than about 0.15 wt. % iron.
 8. A vehicle wheel cast from a chromium-free, zirconium-free aluminum alloy consisting essentially of: about 4-7 wt. % zinc, about 1.2-1.8 wt. % magnesium, at least one of about 0.3-0.6 wt. % manganese and 0.1-0.25 wt. % copper, up to about 0.25 wt. % iron, and up to about 0.25 wt. % silicon, the balance aluminum and incidental impurities, said cast wheel having a typical T6 yield strength of about 50 ksi or higher.
 9. The vehicle wheel of claim 8 which contains about 4-5 wt. % zinc.
 10. The vehicle wheel of claim 8 which contains less than about 0.15 wt. % silicon and less than about 0.15 wt. % iron.
 11. The vehicle wheel of claim 8 wherein said vehicle is a mid-size truck.
 12. A vehicular structural component cast from a chromium-free, zirconium-free aluminum alloy consisting essentially of: about 4-7 wt. % zinc, about 1.2-1.8 wt. % magnesium, at least one of about 0.3-0.6 wt. % manganese and 0.1-0.25 wt. % copper, up to about 0.25 wt. % iron, and up to about 0.25 wt. % silicon, the balance aluminum and incidental impurities, said cast structural component having a T6 tensile yield strength of about 50 ksi or higher.
 13. The vehicular structural component of claim 12 which contains about 4-5 wt. % zinc.
 14. The vehicular structural component of claim 12 which contains less than about 0.15 wt. % silicon and less than about 0.15 wt. % iron.
 15. The vehicular structural component of claim 12 which is a control arm.
 16. The vehicular structural component of claim 12 which is a steering knuckle.
 17. The vehicular structural component of claim 12 which is a differential carrier. 